A recent development of electrophoretic technique is capillary electrophoresis. As with conventional methods of electrophoresis charged molecules may be isolated and determined in an electric field based upon their relative mobilities. A capillary electrophoretic system basically consists of a fused silica capillary having an inner diameter of approximately 25 to 100 microns and which connects two reservoirs filled with a buffer. Separation takes place in the buffer-filled capillary and substances may be detected by UV absorbance or emitted fluorescence by means of a concentrated transverse beam of light passing through the capillary.
In relation to conventional gel electrophoresis the use of a capillary system permits a considerably higher electric field strength due to reduced generation of heat and an improved cooling effect (reduced ratio of cross-sectional to circumferential area). This results in very fast separations with extremely high resolution.
Drawn glass capillary tubes have, however, several disadvantages. Among those may be mentioned the difficulty of providing branched systems as well as the difficulty of creating areas having particular surface characteristics. It is also relatively difficult to manufacture extremely small bore glass tubes. Further, glass capillaries are also unsuitable for parallel channel analyses and only small volumes can therefore be separated, separations for preparative purposes thereby being impractical.
To overcome these disadvantages planar structures have been developed in which a number of trenches or channels are fabricated in parallel. Typically, such a planar structure is produced by etching trenches in a semiconductor substrate, such as a silicon wafer, and then covering the etched surface by a cover plate to complete the electrophoretic channels. Such structures are, however, rather expensive to produce. Further, since the materials used are rigid and hard, it is difficult to provide an adequate sealing between the top edges of the etched trenches and the cover plate. As the etched substrate is most often a semiconductor, the material per se is unsuitable for electrophoretic applications and the channel side-walls must therefore be provided with an insulating surface layer, such as by oxidation or by coating with some other material.